A battery system mounted on an electric vehicle (EV), a plug-in hybrid electric vehicle (PHEV), or a hybrid electric vehicle (HEV) includes a battery controller which detects a voltage, a temperature, and a current of the battery, and calculates a state of charge (SOC) and a state of health (SOH) of the battery based on these detected values in order to improve the performance of the battery at a maximum level while preventing overcharge/overdischarge of the battery of the battery system.
As a method of calculating the SOC of the battery, there is a current integration method in which a capacitance obtained by integrating a current value flowing in the battery and a full charge capacitance of the battery are used. As a method of calculating the SOH of the battery, there is a method in which an internal resistance value is calculated based on a ratio (dV/dI) of a change (dI) of the current flowing in the battery and a change (dV) of a battery voltage, and is compared with the internal resistance when the battery is new so as to calculate the SOH.
The SOH of the battery can be utilized as a criterion of replacement of the battery system. The following PTL 1 discloses a technology in which the SOH obtained through the calculation is compared with a predetermined threshold, and a warning lamp is lit on in a case where there is a need to replace the battery.